FIG. 1 is a schematic diagram of an external clamp-on type ultrasonic flow meter for measuring fluid flow within pipes. The ultrasonic flow meter 2 comprises two ultrasonic transducers, an upstream ultrasonic transducer 4 and a downstream ultrasonic transducer 6 wherein each ultrasonic transducer is provided with a piezoelectric chip 8 for transmitting and receiving ultrasonic signals across a pipe 10. The transducer 6 may also be referred to herein as an ultrasonic transceiver. The principle of time-difference ultrasonic measurement is used in which ultrasonic propagation in the downstream direction demonstrates acoustic velocity increase, and in the counter current direction it is reduced. This relationship is described by the following equation:
  V  =                              (                      M            +            1                    )                *        D                    sin        ⁢                                  ⁢        2        ⁢                                  ⁢        θ              ×                  Δ        ⁢                                  ⁢        T                              T          up                *                  T          down                    wherein V is average flow velocity over the acoustic path, M is ultrasonic signal reflection time, D is pipe inner diameter, θ is the angle between signal and flow, Tup is the signal transmit time from downstream to upstream, Tdown is the signal transmit time from upstream to downstream, and ΔT=Tup−Tdown. When ΔT has a negative value, the fluid flow rate is in the reverse direction, giving the time-difference ultrasonic flow meter utility in bi-directional flow applications.
In addition, the flow velocity of a fluid is different at the different positions of the pipe. The flow velocity in the center of the pipe is faster than that near the tube wall. The velocity distribution of a fluid in a pipe can be represented by a velocity profile. By setting up the flow meter and taking into account the influence of the velocity distribution, the average flow velocity over the cross-sectional area of the pipe can be calculated, and the volume flow rate of the fluid can be obtained according to the cross-sectional area of the pipeline. The calculation formula of volume flow rate (Q) is:Q=Vave×A wherein V is average velocity as above, and A is cross-sectional area. Mass flow rate (M) is calculated:M=ρ×Q wherein ρ is fluid density.
Adapting these principles while accounting for the effect of the pipe on flow rate measurement with given distances s, d, S1 and S2 and angles θ1, θ2, θ3 as understood by those with routine skill in the art, FIG. 2 illustrates the so-called “V-type” for ultrasonic flow meter 12 installation. As shown in FIG. 2, ‘L1’ refers to pipe wall thickness for a pipe of uniform wall thickness. ‘L2’ refers to pipe diameter of a constant-diameter pipe section where the flow meter is installed. ‘θ1’ is the angle of ultrasonic emission relative to the pipe surface normal where the emitted waves impinge on the pipe (the “surface normal”). ‘θ2’ is the angle of transmission of the ultrasonic beam through the pipe wall relative to the surface normal, which can differ from ‘θ1’ due to refraction. ‘θ3’ is the angle of transmission of the beam through the fluid flowing in the pipe relative to the surface normal. ‘d’ is the distance between the ultrasonic transceivers 6 and ‘s’ is the distance between the point of entry and exit of the ultrasonic beam. ‘S1’ is given by ‘L1’ times the tangent of ‘θ2’ and ‘S2’ is given by ‘L2’ times the tangent of ‘θ3’.
Chinese Patent No. 203848888 describes an ultrasonic flow meter adapted to fit different pipe diameters that uses transceivers mounted with housing sections that are adjustable through a range of angles at discrete intervals. To achieve proper angular orientation of the acoustic beams, the distance between the angled transceivers must also be adjusted in many instances because of the gross nature of the angular adjustment. While this complication is touted as a feature allowing for smaller distance achieved between the transducers when used with smaller pipes, the need for lateral sensor adjustment introduces complexity to system setup. Accordingly, a need persists for an improved method for ultrasonic flow measurement that can be adjusted in the field for different pipe sizes, and an improved flow measurement apparatus capable of performing the new method.